Transient enhanced diffusion (TED), particularly of shallow doped layers, such as a boron layer, in a silicon substrate of a semiconductor device, is a known problem.
When a shallow doped layer or region, such as a boron layer, is formed in a semiconductor substrate by ion implantation, the junction depth is not just dependent on the ion implant energy but can also depend on channelling and phenomena such as TED when the implanted ions migrate through the crystal lattice during subsequent thermal processing. It is known to use pre-amorphisation techniques to amorphise the semiconductor substrate by, for example, ion implantation using non-electrically active ions, such as silicon, germanium and fluorine, in order to eliminate channelling. However, pre-amorphisation implantation creates in the substrate an amorphous surface layer adjacent the underlying crystalline semiconductor material and produces a large number of defects beyond the amorphous/crystalline (a/c) interface. These crystal defects are usually called End of Range (EOR) defects. Defects of this kind are known to enhance diffusion of previously implanted dopant ions during subsequent thermal processes of annealing and activation of the semiconductor device. It is also known that during the heat treatment (for annealing and activation), the amorphised layer re-crystallises and the EOR defects dissolve semiconductor interstitials that effectively migrate towards the surface of the structure, so that they become present in the surface doped layer to provide a mechanism for TED.
As is known, TED increases the diffusivity of the dopant in the doped layer with the result that the depth of the shallow doped layer is increased. With the desire to reduce the size of semiconductor devices, several techniques have been proposed to reduce the effects of TED so as to reduce the depth of the doped layer by reducing the EOR defects.
WO 03/049163 describes reducing or eliminating TED by providing a layer rich in a trap element located between a surface implanted boron layer, and the EOR defects beyond the amorphous/crystalline (a/c) interface. Then, during heat treatment, migrating defects are essentially halted or trapped by this layer and cannot migrate up to the surface to provide the TED mechanism in the boron layer. As a result, a junction can be formed in the substrate which is shallower and can have a steeper profile.
Although the above PCT publication suggests that the layer of trapping element (typically carbon atoms) can be introduced by the implantation before or after an amorphisation implantation, the embodiment described in the application in detail uses molecular beam epitaxi (MBE) to grow a carbon enriched silicon layer on a silicon substrate, which is then covered with a further layer of pure silicon. The PCT application also suggests that it is important to avoid the presence of carbon atoms in the active boron junction region to avoid impairment of the electrical properties of the junction.
The present invention proposes an implantation procedure which provides an advantage of reducing TED by purely implant techniques.